1. Field of the Invention
The invention is directed to a micro-metering device for the volumetric or gravimetric continuous delivery of bulk materials which comprises a bulk material container, discharge means and a metering screw.
In this connection, bulk materials can be divided into three main groups:
bulk materials having light flow properties are those which can flow freely without auxiliary means through a base opening having a diameter of five times the largest kernel size, e.g. granulate, cereals, etc. PA1 bulk materials having heavy flow properties which build bridges over the outlet opening, e.g. the most flour-like, i.e. fine-grained products. A greater discharge surface must be utilized with these products. PA1 bulk materials having medium flow properties which are assigned to one of the first two groups depending on the physical conditions, e.g. moisture, storage time, etc.
2. Discussion of Related Art
In the preparation of foodstuffs and feedstuffs, it is often necessary to mix together mixture components in relatively small quantities or to mix in a small percentage of additives in larger batches. For example, this is the case with soups and seasoning mixtures or in the production of feed mixtures, also, e.g., in the metered addition of flour to semolina in pasta production.
In automatic operations, it is required in every case that the components be combined in volumes or weight proportions which can be exactly determined. Industrial practice demands that operation be as trouble-free as possible.
Known problems of pouring with sugar and salt shakers should be avoided in automatic operations. Two methods are available for avoiding these problems. First, the product to be metered is put into a form such that the material always remains in a virtually pourable state, i.e. always retains light flow properties. This often requires additional treatments such as drying or the production of a special granulation or a powder form which generally amounts to an increase in the cost of the raw material.
The second method consists in making all metering devices more complicated, so that stoppage and bridging are both impossible.
U.S. Pat. No. 3,151,782 discloses a solution which is widespread in various fields, particularly for chemical materials. A common drive ensures both the product feed from a reservoir and the exact metering or movement of a metering screw. The feed from the reservoir is produced by means of a shaking movement, wherein the material is prevented from shooting through the base opening by means of a cover plate over the opening. However, many materials, particularly foodstuffs or feedstuffs, tend to cake together when vibrated, so that the vibrations are not a genuine metering means in this instance. On the contrary, operating reliability is jeopardized by the shaking.